Our cat had been gone for fifteen days before we managed to find him in our neighborhood.
Italics are used in the sentence above because the italicized words are:

Question

Quizplus · Accepted Answer

The word "fifteen" is italicized because the author wanted to emphasize the length of the time cat had been missing. 
The scientific name of an organism is incorrect. The word "fifteen" is italicized because the author wanted to emphasize the length of the time cat had been missing. Nothing in the sentence involves a scientific name of an organism. 
Words that the reader may not know is incorrect. The word "fifteen" is italicized because the author wanted to emphasize the length of the time cat had been missing. Nothing in the sentence involves words that the reader may not know. 
A proper title of something is incorrect. The word "fifteen" is italicized because the author wanted to emphasize the length of the time cat had been missing. Nothing in the sentence involves a proper title of something.

Our Cat Had Been Gone for Fifteen Days Before We